Superabsorbers are water-insoluble, crosslinked polymers which are able to absorb large amounts of aqueous fluids, especially body fluids, more especially urine or blood, with swelling and the formation of hydrogels, and to retain such fluids under a certain pressure. By virtue of those characteristic properties, such polymers are chiefly used for incorporation into sanitary articles, such as, for example, baby's nappies/diapers, incontinence products or sanitary towels.
The preparation of superabsorbers is generally carried out by free-radical polymerisation of acid-group-carrying monomers in the presence of crosslinkers, it being possible for polymers having different absorber properties to be prepared by the choice of the monomer composition, the crosslinkers and the polymerisation conditions and of the processing conditions for the hydrogel obtained after the polymerisation (for details see, for example, Modern Superabsorbent Polymer Technology, F L Buchholz, G T Graham, Wiley-VCH, 1998).
The acid-group-carrying monomers can be polymerized in the presence of the crosslinkers in a batch process or in a continuous process. An example for a continuous process for preparing superabsorbers is disclosed in U.S. Pat. No. 4,857,610. According to this process, an aqueous monomer solution containing acrylic acid and crosslinkers is continuously charged in a layer at least one centimeter thick on a moving endless conveyor belt and polymerized. The gel that is thus obtained is continuously discharged from the conveyor belt and is subsequently comminuted, dried, sized and optionally surface treated.
The surface treatment of the water-absorbent polymer particles usually comprises a surface-crosslinking step, in which the polymer particles are first mixed with an aqueous solution comprising a surface-crosslinking agent and in which the thus obtained mixture is than heat-treated in a further mixing device, preferably in a horizontally operated mixing device such as an electrically driven NARA-mixer. However, in order to obtain water-absorbent polymer particles with predictable and reproducible absorption properties it is very important to control the residence time of the polymer particles in the NARA-mixer. The disadvantage of the prior art processes, however, can be seen in the fact that if the NARA-mixer has to be stopped, for example due to a certain overload or for other reasons, it is very difficult to restart the mixer in the fully loaded state, especially if the fluidity of the a moist polymer powder is decreased due to the presence of additives such as aluminium salts. In case of an electrically driven NARA-mixer the maximum torque can not be reached right from the restart of the mixer, with the result that in the initial phase of the restart the polymer particles can not be mixed sufficiently. These problems in restarting the NARA-mixer result in an unpredictable extension of the residence time of the polymer particles in the NARA-mixer and thus to unpredictable absorption properties of the end product. As the inner surfaces of the NARA-mixer are heated, the polymer particles may even inflame if they contact these surfaces for a prolonged time.
An object of the invention is to overcome at least a part of the above mentioned drawbacks of the prior art. A further object is to provide a process to prepare water-absorbent polymer particles in a time and resource efficient way. A further object of the invention is to perform the process of treating the surface of water-absorbent polymer particles, especially the process of crosslinking the surface of water-absorbent polymer particles, under more reproducible conditions. Another object is to provide a continuous process for treating the surface of water-absorbent polymer particles, especially for crosslinking the surface of water-absorbent polymer particles, which can be performed more efficiently, e.g. which can be stopped and restarted more easily. The process should also allow to homogeneously mix additives, which usually have a negative impact on the fluidity of the mixture, with water-absorbent polymer particles.